1. Field of the Invention
This invention relates generally to systems for driving strings of LEDs, and more particularly to LED driving systems that employ a switching power converter to supply power to the LEDs.
2. Description of the Related Art
A LED is often driven by providing a DC voltage on the LED's anode terminal and an AC signal on its cathode terminal, with the AC signal operating to pulse-width modulate (PWM) the current conducted by the LED. The duty cycle of the PWM signal determines the LED's brightness. This same technique is also used to drive ‘strings’ of LEDs, which consist of multiple LEDs connected in series, with the cathode of one LED connected to the anode of the next LED. In this case, the DC voltage is provided to the first anode in the string, and the AC signal is provided to the last cathode in the string.
The DC voltage provided to the LEDs may be provided by, for example, a switching power converter—most typically a boost-type power converter (referred to herein as a ‘boost converter’, which produces an output referred to herein as a ‘boost voltage’). A boost converter includes an inductor coupled to an input voltage, a switching element, an output terminal coupled to the inductor, and a controller arranged to operate the switching element so as to control the flow of current in the inductor and thereby provide a desired boost voltage at the output terminal.
Such an arrangement is shown in FIG. 1a. A boost converter 10 comprises an inductor L1 connected between an input voltage Vin and the anode of a diode D1, the cathode of which is connected to an output terminal 12; an output capacitor C1 is also connected to output terminal 12. A switching element S1 is connected to the node between L1 and D1, and operated by a controller 14 which receives a signal which varies with the voltage at output terminal 12 (Vout) and operates S1 (via a control signal BST) as needed to achieve a desired Vout value. A string 16 of LEDs may be connected between output terminal 12 and a switching circuit 18, which is controlled by an AC signal LED ON to pulse-width modulate the current in string 16, such that the brightness of the LEDs varies with the duty cycle of LED ON.
To conserve power in a LED driving system of the sort shown in FIG. 1a, the boost converter's switching element may only be actively switched when LED ON is in the state necessary to cause the LEDs in string 16 to be on. This is illustrated in the timing diagram shown in FIG. 1b. In this example, the LEDs are turned on when LED ON goes high, causing the current in the string (ILED) to increase. This is also when switch control signal BST is active.
As noted above, the brightness of the LEDs is determined by the duty cycle of LED ON. However, in a system such as that shown in FIG. 1a, a problem can arise if the duty cycle of LED ON is too low. In this case, the number of consecutive switching cycles for signal BST will also be low. As the energy stored in L1 increases with the number of consecutive switching cycles, the relatively small number of cycles that can occur when the LED ON duty cycle is low can result in the energy being stored in L1 being insufficient to support the desired boost voltage. This causes the boost voltage to ‘collapse’—i.e., be below the desired value. This effect serves to limit the minimum duty cycle for LED ON, and thereby limits the contrast ratio achievable with the LED driving system.